Systems and methods for voltage control and current control of power conversion systems with multiple operation modes

ABSTRACT

System and method for regulating a power conversion system. A system controller for regulating a power conversion system includes an operation-mode-selection component and a driving component. The operation-mode-selection component is configured to receive a first signal related to an output load of the power conversion system and a second signal related to an input signal received by the power conversion system and output a mode-selection signal based on at least information associated with the first signal and the second signal. The driving component is configured to receive the mode-selection signal and generate a drive signal based on at least information associated with the mode-selection signal, the driving signal corresponding to a switching frequency.

1. CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to Chinese Patent Application No.201210342097.5, filed Sep. 14, 2012, incorporated by reference hereinfor all purposes.

2. BACKGROUND OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides systems and methods for voltageregulation and current regulation. Merely by way of example, theinvention has been applied to a power conversion system. But it would berecognized that the invention has a much broader range of applicability.

FIG. 1 is a simplified diagram showing a conventional flyback powerconversion system. The power conversion system 100 includes a primarywinding 102, a secondary winding 104, an auxiliary winding 114, a powerswitch 106, a current sensing resistor 108, two rectifying diodes 110and 116, two capacitors 112 and 118, three resistors 120, 122 and 124,and a system controller 160. For example, the power switch 106 is abipolar transistor. In another example, the power switch 106 is a MOStransistor.

As shown in FIG. 1, the power conversion system 100 uses a transformerincluding the primary winding 102 and the secondary winding 104 toisolate a primary side and a secondary side of the power conversionsystem 100. Information related to an output voltage 126 on thesecondary side can be extracted through the auxiliary winding 114 and afeedback signal 154 is generated based on information related to theoutput voltage 126. The controller 160 receives the feedback signal 154,and generates a drive signal 156 to turn on and off the switch 106 inorder to regulate the output voltage 126.

When the power switch 106 is closed (e.g., on), the energy is stored inthe transformer including the primary winding 102 and the secondarywinding 104. Then, when the power switch 106 is open (e.g., off), thestored energy is released to the output terminal, and the voltage of theauxiliary winding 114 maps the output voltage 126 as follows.

$\begin{matrix}{V_{FB} = {{\frac{R_{2}}{R_{1} + R_{2}} \times V_{aux}} = {k \times n \times \left( {V_{O} + V_{F} + {I_{O} \times R_{eq}}} \right)}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

where V_(FB) represents the feedback signal 154, V_(aux) represents avoltage 158 of the auxiliary winding 114, R₁ and R₂ represent theresistance values of the resistors 122 and 124 respectively. Inaddition, k represents a feedback coefficient, n represents a turnsratio of the secondary winding 104 and the auxiliary winding 114, andR_(eq) represents a cable resistance 120. Further, V_(O) represents theoutput voltage 126, To represents an output current 128, and V_(F)represents a forward voltage of the rectifying diode 110.

A switching period of the switch 106 includes an on-time period duringwhich the switch 106 is closed (e.g., on) and an off-time period duringwhich the switch 106 is open (e.g., off). For example, in adiscontinuous conduction mode (DCM), a next switching cycle does notstart until a period of time after the completion of a demagnetizationprocess associated with the transformer including the primary winding102 and the secondary winding 104. In another example, in a continuousconduction mode (CCM), a next switching cycle starts before thecompletion of the demagnetization process. Thus, the actual length ofthe demagnetization process before the next switching cycle starts islimited to the off-time period of the switch 106. In yet anotherexample, in a quasi-resonant (QR) mode or a critical conduction mode(CRM), a next switching cycle starts shortly after the completion of thedemagnetization process. FIG. 2(A), FIG. 2(B) and FIG. 2(C) aresimplified conventional timing diagrams for the power conversion system100 that operates in the DCM mode, in the CCM mode, and the QR mode(e.g., the CRM mode), respectively.

FIG. 2(A) is a simplified conventional timing diagram for the flybackpower conversion system 100 that operates in the discontinuousconduction mode (DCM). The waveform 170 represents the voltage 158 ofthe auxiliary winding 114 as a function of time, and the waveform 172represents a secondary current 162 flowing through the secondary winding104 as a function of time. Three time periods are shown in FIG. 2(A),including an on-time period Torr, an off-time period T_(off) and ademagnetization period T_(Demag). For example, T_(on) starts at time t₀and ends at time t₁, T_(Demag) starts at the time t₁ and ends at timet₃, and T_(off) starts at the time t₃ and ends at time t₄. In anotherexample, t₀≤t₁≤t₂≤t₃≤t₄.

The controller 160 often implements a sample-and-hold mechanism. Whenthe demagnetization process on the secondary side of the powerconversion system 100 is almost completed (e.g., at t₃), the secondarycurrent 162 becomes almost zero (e.g., as shown by the waveform 172).The voltage 158 of the auxiliary winding 114 is usually sampled at t₂(e.g., point A). The sampled voltage value is often held until thevoltage 158 is sampled again during a next demagnetization period.Through a negative feedback loop, the sampled voltage value can becomeequal to a reference voltage V_(ref) as follows:

V _(FB) =V _(ref)  (Equation 2)

Thus, the output voltage 126 can be determined as follows:

$\begin{matrix}{V_{O} = {\frac{V_{ref}}{k \times n} - V_{F} - {I_{O} \times R_{eq}}}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

As shown in FIG. 2(A), after the demagnetization process ends (e.g., att₃), one or more valleys (e.g., the valleys 180, 181 and 182) appear inthe voltage 158 of the auxiliary winding 114 (e.g., as shown by thewaveform 170) before the start of a next switching cycle, as an example.In another example, the power conversion system 100 operates in a valleyskipping mode. That is, the next switching cycle is triggered by avalley other than the first valley (e.g., the valley 180).

FIG. 2(B) is a simplified conventional timing diagram for the flybackpower conversion system 100 that operates in the continuous conductionmode (CCM). The waveform 202 represents the voltage 158 of the auxiliarywinding 114 as a function of time, the waveform 204 represents asecondary current 162 flowing through the secondary winding 104 as afunction of time, and the waveform 206 represents a primary current 164flowing through the primary winding 102 as a function of time. Threetime periods are shown in FIG. 2(B), including an on-time period T_(on),an off-time period T_(off) and a demagnetization period T_(Demag). Forexample, T_(on) starts at time t₅ and ends at time t₆, T_(Demag) startsat the time t₆ and ends at time t₈, and T_(off) starts at the time t₆and ends at the time t₈. In another example, t₅≤t₆≤t₇≤t₈.

FIG. 2(C) is a simplified conventional timing diagram for the flybackpower conversion system 100 that operates in the quasi-resonant (QR)mode (e.g., the CRM mode). The waveform 208 represents the voltage 158of the auxiliary winding 114 as a function of time, the waveform 210represents a secondary current 162 flowing through the secondary winding104 as a function of time, and the waveform 212 represents a primarycurrent 164 flowing through the primary winding 102 as a function oftime. In addition, the waveform 214 represents an internal signal of thecontroller 160 associated with the demagnetization process as a functionof time, and the waveform 216 represents the drive signal 156 as afunction of time.

Three time periods are shown in FIG. 2(C), including an on-time periodT_(on), an off-time period T_(off) and a demagnetization periodT_(Demag). For example, T_(on) starts at time t₉ and ends at time t₁₀,T_(Demag) starts at the time t₁₀ and ends at time t₁₂, and T_(off)starts at the time t₁₀ and ends at the time t₁₃. In another example,t₉≤t₁₀≤t₁₁≤t₁₂≤t₁₃.

For example, the power conversion system 100 operates in a valleyswitching mode. That is, after the demagnetization process ends (e.g.,at t₁₂), a next switching cycle is triggered when the power conversionsystem 100 detects a first valley (e.g., the valley 220) in the voltage158 of the auxiliary winding 114 (e.g., as shown by the waveform 208).

As discussed above, the power conversion system 100 can operate in theDCM mode, the CCM mode, or the QR mode (e.g., the CRM mode and/or thevalley switching mode). However, when operating in a single mode, thepower conversion system 100 often does not have a satisfactoryefficiency under certain circumstances. Hence, it is highly desirable toimprove techniques for voltage regulation and current regulation of apower conversion system.

3. BRIEF SUMMARY OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides systems and methods for voltageregulation and current regulation. Merely by way of example, theinvention has been applied to a power conversion system. But it would berecognized that the invention has a much broader range of applicability.

According to one embodiment, a system controller for regulating a powerconversion system includes an operation-mode-selection component and adriving component. The operation-mode-selection component is configuredto receive a first signal related to an output load of the powerconversion system and a second signal related to an input signalreceived by the power conversion system and output a mode-selectionsignal based on at least information associated with the first signaland the second signal. The driving component is configured to receivethe mode-selection signal and generate a drive signal based on at leastinformation associated with the mode-selection signal, the drivingsignal corresponding to a switching frequency. Theoperation-mode-selection component is further configured to, if theoutput load is determined to be larger than a first load threshold inmagnitude and the input signal is determined to be larger than an inputthreshold, generate the mode-selection signal corresponding to aquasi-resonant mode if the switching frequency is determined to besmaller than a first frequency threshold and larger than a secondfrequency threshold. In addition, the operation-mode-selection componentis configured to, if the output load is determined to be larger than thefirst load threshold in magnitude and the input signal is determined tobe smaller than the input threshold, generate the mode-selection signalcorresponding to a continuous conduction mode if the switching frequencyis determined to be equal to the second frequency threshold.

According to another embodiment, a system controller for regulating apower conversion system includes a driver component, a firstsample-and-hold component, a second sample-and-hold component, a firstswitch, a second switch, and a signal processing component. The drivercomponent is configured to output a drive signal in order to affect anoutput signal of the power conversion system, the drive signal beingassociated with a switching frequency corresponding to a switchingperiod, the switching period including an on-time period and ademagnetization period. The first sample-and-hold component includes afirst capacitor and is configured to sample and hold a current sensingsignal associated with a primary current flowing through a primarywinding of the power conversions system at at least a first time duringthe on-time period and generate a first held sampled signal based on atleast information associated with the current sensing signal. The secondsample-and-hold component includes a second capacitor and is configuredto sample and hold the current sensing signal at at least a second timeduring the on-time period and generate a second held sampled signalbased on at least information associated with the current sensingsignal, the second time being later than the first time. The firstswitch includes a first switch terminal and a second switch terminal,the first switch terminal being coupled to the first capacitor, thesecond switch terminal being coupled to the second capacitor, the firstswitch being further configured to be closed during the demagnetizationperiod and open during the on-time period. The second switch includes athird switch terminal and a fourth switch terminal, the third switchterminal being coupled to the first switch terminal, the second switchbeing further configured to be closed during the demagnetization periodand open during the on-time period. The signal processing component isconfigured to receive a combined signal from the fourth switch terminalif the first switch and the second switch are closed and output aprocessed signal based on at least information associated with thecombined signal to the driver component.

According to yet another embodiment, a system controller for regulatinga power conversion system includes a driver component, a sample-and-holdcomponent, a switch, a signal processing component, and anoperation-mode-selection component. The driver component is configuredto output a drive signal in order to affect an output signal of thepower conversion system, the drive signal being associated with aswitching frequency corresponding to a switching period, the switchingperiod including an on-time period and a demagnetization period. Thesample-and-hold component includes a first capacitor and is configuredto sample and hold a current sensing signal associated with a primarycurrent flowing through a primary winding of the power conversionssystem at a middle point of the on-time period and generate a heldsampled signal based on at least information associated with the currentsensing signal. The switch includes a first switch terminal and a secondswitch terminal, the first switch terminal being coupled to the firstcapacitor, the switch being further configured to be closed during thedemagnetization period and open during the on-time period. The signalprocessing component is configured to receive a third signal from thesecond switch terminal if the switch is closed and output a processedsignal based on at least information associated with the third signal tothe driver component. The operation-mode-selection component isconfigured to receive a first signal related to an output load of thepower conversion system and a second signal related to an input signalreceived by the power conversion system and output a mode-selectionsignal based on at least information associated with the first signaland the second signal. The driver component is further configured toreceive the mode-selection signal and generate the drive signal based onat least information associated with the mode-selection signal.

In one embodiment, a method for regulating a power conversion systemincludes receiving a first signal related to an output load of the powerconversion system and a second signal related to an input signalreceived by the power conversion system, processing informationassociated with the first signal and the second signal, and generating amode-selection signal based on at least information associated with thefirst signal and the second signal. In addition, the method includesreceiving the mode-selection signal, processing information associatedwith the mode-selection signal, and generating a drive signal based onat least information associated with the mode-selection signal. Theprocess for generating a mode-selection signal based on at leastinformation associated with the first signal and the second signalincludes if the output load is determined to be larger than a first loadthreshold in magnitude and the input signal is determined to be largerthan an input threshold, generating the mode-selection signalcorresponding to the quasi-resonant mode if the switching frequency isdetermined to be smaller than a first frequency threshold and largerthan a second frequency threshold. The process for generating amode-selection signal based on at least information associated with thefirst signal and the second signal further includes if the output loadis determined to be larger than the first load threshold in magnitudeand the input signal is determined to be smaller than the inputthreshold, generating the mode-selection signal corresponding to thecontinuous conduction mode if the switching frequency is determined tobe equal to the second frequency threshold.

In another embodiment, a method for regulating a power conversion systemincludes generating a drive signal in order to affect an output signalof the power conversion system, the drive signal being associated with aswitching frequency corresponding to a switching period, the switchingperiod including an on-time period and a demagnetization period, andsampling and holding, by at least a first sample-and-hold component, atat least a first time during the on-time period, a current sensingsignal in order to generate a first held sampled signal, the currentsensing signal being associated with a primary current flowing through aprimary winding of the power conversions system, the firstsample-and-hold component including a first capacitor. The methodfurther includes sampling and holding, by at least a secondsample-and-hold component, at at least a second time during the on-timeperiod, the current sensing signal in order to generate a second heldsampled signal, the second sample-and-hold component including a secondcapacitor, the second time being later than the first time. In addition,the method includes generating a combined signal during thedemagnetization period by at least a first switch, the first switchincluding a first switch terminal coupled to the first capacitor and asecond switch terminal coupled to the second capacitor, receiving thecombined signal by at least a second switch including a third switchterminal and a fourth switch terminal, the third switch terminal beingcoupled to the first switch terminal, and outputting a processed signalbased on at least information associated with the combined signal duringthe demagnetization period.

In yet another embodiment, a method for regulating a power conversionsystem includes receiving a first signal related to an output load ofthe power conversion system and a second signal related to an inputsignal received by the power conversion system, processing informationassociated with the first signal and the second signal, and generating amode-selection signal based on at least information associated with thefirst signal and the second signal. The method further includesreceiving the mode-selection signal, processing information associatedwith the mode-selection signal, and generating the drive signal based onat least information associated with the mode-selection signal in orderto affect an output signal of the power conversion system, the drivesignal being associated with a switching frequency corresponding to aswitching period, the switching period including an on-time period and ademagnetization period. In addition, the method includes sampling andholding, by at least a sample-and-hold component, at a middle point ofthe on-time period, a current sensing signal in order to generate a heldsampled signal, the current sensing signal being associated with aprimary current flowing through a primary winding of the powerconversions system, the sample-and-hold component including a capacitor,receiving a third signal during the demagnetization period through atleast a switch coupled to the capacitor, and outputting a processedsignal based on at least information associated with the third signalduring the demagnetization period.

Many benefits are achieved by way of the present invention overconventional techniques. Certain embodiments of the present inventionprovide systems and methods to employ multiple operation modes so that apower conversion system operates in a discontinuous conduction modeunder no/light load conditions, operates in a quasi-resonant mode undermedium load conditions, and operates, under full/heavy load conditions,in a continuous conduction mode for a low line input voltage or in thequasi-resonant mode for a high line input voltage in order to improvethe overall efficiency of the power conversion system. Some embodimentsof the present invention provide systems and methods to operate thepower conversion system in a valley switching mode for a high line inputvoltage to reduce the switching loss and improve the system efficiency.Certain embodiments of the present invention provide systems and methodsto operate the power conversion system in a continuous conduction modefor a low line input voltage to reduce the conduction loss and improvethe system efficiency. Some embodiments of the present invention providesystems and methods to operate the power conversion system in afrequency reduction mode (e.g., the discontinuous conduction mode or avalley skipping mode) under no/very light load conditions to reduce theswitching loss and improve the system efficiency. Certain embodiments ofthe present invention provide systems and methods to operate the powerconversion system in a quasi-resonant mode for a medium-high inputvoltage and/or under light load conditions to reduce the switching loss.Some embodiments of the present invention provide systems and methods tooperate the power conversion system in a fixed frequency mode (e.g.,CCM) for a low input voltage and/or under a full/heavy load conditionsto reduce the conduction loss and improve the system efficiency.

Depending upon embodiment, one or more of these benefits may beachieved. These benefits and various additional objects, features andadvantages of the present invention can be fully appreciated withreference to the detailed description and accompanying drawings thatfollow.

4. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram showing a conventional flyback powerconversion system.

FIG. 2(A) is a simplified conventional timing diagram for the powerconversion system as shown in FIG. 1 that operates in the discontinuousconduction mode (DCM).

FIG. 2(B) is a simplified conventional timing diagrams for the powerconversion system as shown in FIG. 1 that operates in the continuousconduction mode (CCM).

FIG. 2(C) is a simplified conventional timing diagrams for the powerconversion system as shown in FIG. 1 that operates in the quasi-resonant(QR) mode.

FIG. 3(A) is a simplified diagram showing a power conversion system witha controller according to an embodiment of the present invention.

FIG. 3(B) is a simplified diagram showing a switching frequency of thepower conversion system as shown in FIG. 3(A) and a primary currentflowing through the primary winding of the power conversion system asshown in FIG. 3(A) according to an embodiment of the present invention.

FIG. 3(C) is a simplified diagram showing a switching frequency of thepower conversion system as shown in FIG. 3(A) and a primary currentflowing through the primary winding of the power conversion system asshown in FIG. 3(A) according to another embodiment of the presentinvention.

FIG. 3(D) is a simplified diagram showing multiple operation modes ofthe power conversion system as shown in FIG. 3(A) being determined basedon the output load and the input voltage as shown in FIG. 3(B) accordingto an embodiment of the present invention.

FIG. 4(A) is a simplified diagram showing certain components of thecontroller as part of the power conversion system as shown in FIG. 3(A)according to one embodiment of the present invention.

FIG. 4(B) is a simplified diagram showing certain components of themulti-mode controller as part of the controller as shown in FIG. 3(A)according to one embodiment of the present invention.

FIG. 5 is a simplified diagram showing certain components of thecontroller as part of the power conversion system as shown in FIG. 3(A)according to another embodiment of the present invention.

FIG. 6 is a simplified diagram showing certain components of thecontroller as part of the power conversion system as shown in FIG. 3(A)according to yet another embodiment of the present invention.

FIG. 7 is a simplified timing diagram for the power conversion systemincluding the controller as shown in FIG. 6 that operates in thecontinuous conduction mode (CCM) according to an embodiment of thepresent invention.

FIG. 8 is a simplified diagram showing certain components of thecontroller as part of the power conversion system as shown in FIG. 3(A)according to yet another embodiment of the present invention.

FIG. 9 is a simplified diagram showing certain components of thecontroller as part of the power conversion system as shown in FIG. 3(A)according to yet another embodiment of the present invention.

FIG. 10 is a simplified timing diagram for the power conversion systemincluding the controller as shown in FIG. 9 that operates in thecontinuous conduction mode (CCM) according to an embodiment of thepresent invention.

5. DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides systems and methods for voltageregulation and current regulation. Merely by way of example, theinvention has been applied to a power conversion system. But it would berecognized that the invention has a much broader range of applicability.

Referring to FIG. 1, power loss in the power conversion system 100 oftenincludes a switching loss and a conduction loss. The conduction lossusually is associated with an on-resistance of the power switch 106. Forexample, when the power conversion system 100 receives a high line inputvoltage, the switching loss contributes more to the power loss than theconduction loss. In another example, when the power conversion system100 receives a low line input voltage, the conduction loss contributesmore to the power loss than the switching loss if the output load isfull/heavy. Accordingly, multiple operation modes can be implemented inorder to reduce the power loss of power conversion systems with variousload conditions and/or input voltages.

FIG. 3(A) is a simplified diagram showing a power conversion system witha controller according to an embodiment of the present invention. Thisdiagram is merely an example, which should not unduly limit the scope ofthe claims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. The power conversion system300 includes a primary winding 302, a secondary winding 304, anauxiliary winding 314, a power switch 306, a current sensing resistor308, two rectifying diodes 310 and 316, two capacitors 312 and 318,three resistors 320, 322 and 324, and a system controller 360. Forexample, the power switch 306 is a bipolar transistor. In anotherexample, the power switch 306 is a MOS transistor.

According to one embodiment, the power conversion system 300 uses atransformer including the primary winding 302 and the secondary winding304 to isolate a primary side and a secondary side of the powerconversion system 300. For example, the power conversion system 300receives an input voltage 370 on the primary side. In another example,information related to an output voltage 326 on the secondary side canbe extracted through the auxiliary winding 314 and a feedback signal 354is generated based on information related to the output voltage 326. Inanother example, the controller 360 receives the feedback signal 354,and generates a drive signal 356 to turn on and off the switch 306 inorder to regulate the output voltage 326. In yet another example, theoperation mode (e.g., DCM, CCM, QR) of the power conversion system 300is affected by the controller 360.

FIG. 3(B) is a simplified diagram showing a switching frequency of thepower conversion system 300 and a primary current 364 flowing throughthe primary winding 302 of the power conversion system 300 according toan embodiment of the present invention. This diagram is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. The waveform 382 represents the switching frequencyof the power conversion system 300 as a function of output load when theinput voltage 370 is a high line input voltage (e.g., close to 264V AC),and the waveform 388 represents the primary current 364 of the powerconversion system 300 as a function of the output load when the inputvoltage 370 is the high line input voltage.

Five regions are shown in FIG. 3(B), including region I, region II,region III, region IV, and region V. For example, as shown in FIG. 3(B),region I represents an output load range of L₀ and L₁, region IIrepresents an output load range of L₁ and L₂, and region III representsan output load range of L₂ and L₆. In another example, region IVrepresents an output load range of L₆ and L₈, and region V is representsan output load range of L₈ and L₉. In yet another example,L₀≤L₁≤L₂≤L₃≤L₄≤L₆≤L₇≤L₈≤L₉ in magnitude.

According to one embodiment, as shown in FIG. 3(B), if the powerconversion system 300 is under no/light/medium load conditions (e.g., inregions I, II and/or III), the system 300 operates in a DCM mode. Forexample, if the output load is within region I (e.g., no/very lightload), the primary current 364 is kept at a low magnitude 394 (e.g.,IS_min) as shown by the waveform 388. For example, the switchingfrequency increases as the output load increases (e.g., as shown by thewaveform 382). The system 300 operates in a pulse-frequency-modulation(PFM) mode, e.g., a frequency-reduction mode, in some embodiments. Forexample, if the output load is within region II (e.g., light load), theswitching frequency is kept at a magnitude 390 (e.g., as shown by thewaveform 382). In another example, the primary current 364 increases inmagnitude as the output load increases (e.g., as shown by the waveform388). In yet another example, a turn-on time for the switch 306increases for a given input voltage. The system 300 operates in apulse-width-modulation (PWM) mode in certain embodiments. For example,as shown in FIG. 3(B), if the output load is within region III (e.g.,medium load), the primary current 364 continues to increase in magnitudeas the output load increases if the output load is smaller than L₃ inmagnitude, and then is kept at a magnitude 392 within the load rangebetween L₃ and L₄ (e.g., as shown by the waveform 388). In anotherexample, if the output load is larger than L₄ in magnitude, the primarycurrent 364 b increases in magnitude as the output load increases (e.g.,as shown by the waveform 388). In yet another example, the switchingfrequency increases as the output load increases until reaching amaximum frequency value 396 (e.g., as shown by the waveform 382). Thesystem 300 operates in a valley skipping mode in which a switching cycleis triggered when the system 300 detects a valley other than a firstvalley in a voltage 358 of the auxiliary winding 314 according tocertain embodiments.

According to another embodiment, as shown in FIG. 3(B), if the outputload is within region IV, the primary current 364 increases in magnitudeas the output load increases (e.g., as shown by the waveform 388). Forexample, if the switching frequency reaches and keeps at the maximumfrequency value 396 (e.g., in the output load range of L₆ and L₇), thepower conversion system 300 operates in the DCM mode. In anotherexample, if the switching frequency decreases from the maximum frequencyvalue 396 as the output load increases (e.g., in the output load rangeof L₇ and L₈), the power conversion system 300 operates in the QR modeor the valley switching mode. That is, a switching cycle is triggeredwhen the system 300 detects a valley (e.g., a first valley) in a voltage358 of the auxiliary winding 314. According to yet another embodiment,if the output load is within region V, the primary current 364 continuesto increase in magnitude as the output load increases (e.g., as shown bythe waveform 388). For example, the switching frequency decreases as theoutput load increases (e.g., as shown by the waveform 382), and thesystem 300 operates in the QR mode or the valley switching mode.

FIG. 3(C) is a simplified diagram showing a switching frequency of thepower conversion system 300 and a primary current 364 flowing throughthe primary winding 302 of the power conversion system 300 according toanother embodiment of the present invention. This diagram is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. The waveform 382 represents the switching frequencyof the power conversion system 300 as a function of output load when theinput voltage 370 is a high line input voltage (e.g., close to 264V AC),the waveform 384 represents the switching frequency of the powerconversion system 300 as a function of the output load when the inputvoltage 370 is a low line input voltage (e.g., close to 90V AC), thewaveform 386 represents the primary current 364 of the power conversionsystem 300 as a function of the output load when the input voltage 370is the low line input voltage, and the waveform 388 represents theprimary current 364 of the power conversion system 300 as a function ofthe output load when the input voltage 370 is the high line inputvoltage.

FIG. 3(D) is a simplified diagram showing multiple operation modes ofthe power conversion system 300 being determined based on the outputload and the input voltage 370 according to an embodiment of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications. For example,as shown in FIG. 3(D), the input voltage 370 received by the system 300is between a high line input voltage (e.g., V₂) and a low line inputvoltage (e.g., V₀). In yet another example, V₀≤V₁≤V₂.

According to one embodiment, as shown in FIG. 3(B) and FIG. 3(C), if thepower conversion system 300 is under no/light load conditions (e.g., inregions I and/or II), the system 300 operates in a DCM mode regardlessof the magnitude of the input voltage 370. For example, if the outputload is within region I (e.g., no/very light load), the primary current364 is kept at a low magnitude 394 (e.g., IS_min) as shown by thewaveforms 386 and 388. For example, the switching frequency increases asthe output load increases (e.g., as shown by the waveforms 382 and 384).The system 300 operates in a pulse-frequency-modulation (PFM) mode,e.g., a frequency-reduction mode, in some embodiments. For example, ifthe output load is within region II (e.g., light load), the switchingfrequency is kept at a magnitude 390 (e.g., as shown by the waveforms382 and 384). In another example, the primary current 364 increases inmagnitude as the output load increases (e.g., as shown by the waveforms386 and 388). In yet another example, a turn-on time for the switch 306increases for a given input voltage. The system 300 operates in apulse-width-modulation (PWM) mode in certain embodiments.

According to another embodiment, as shown in FIG. 3(C), if the outputload is within region III (e.g., medium load), the primary current 364continues to increase in magnitude as the output load increases if theoutput load is smaller than L₃ in magnitude, and then is kept at amagnitude 392 within the load range between L₃ and L₄ (e.g., as shown bythe waveforms 386 and 388). For example, if the output load is largerthan L₄ in magnitude, the primary current 364 increases in magnitude asthe output load increases, and increases faster when the input voltage370 is close to the low line input voltage (e.g., V₀) than when theinput voltage 370 is close to the high line input voltage (e.g., V₂) asshown by the waveforms 386 and 388. In another example, when the inputvoltage 370 is at the high line input voltage (e.g., V₂), the switchingfrequency increases in magnitude until reaching a maximum frequencyvalue 396 (e.g., at L₆), the power conversion system 300 operates in theDCM mode. In another example, when the input voltage 370 is at the lowline input voltage (e.g., V₀), the switching frequency increases inmagnitude until reaching a maximum frequency value 397 (e.g., at L₅),and the power conversion system operates in the DCM mode. The borderbetween region III and region IV varies according to the magnitude ofthe input voltage 370 in some embodiments. For example, when the inputvoltage 370 is at the high line input voltage (e.g., V₂), the borderbetween region III and region IV is at the output load L₆. In anotherexample, when the input voltage 370 is at the low line input voltage(e.g., V₀), the border between region III and region IV is at the outputload L₅.

According to yet another embodiment, as shown in FIG. 3(C), if theoutput load is within region IV, the primary current 364 continues toincrease in magnitude as the output load increases, and increases fasterwhen the input voltage 370 is close to the low line input voltage thanwhen the input voltage 370 is close to the high line input voltage(e.g., as shown by the waveforms 386 and 388). For example, when theinput voltage 370 is at the high line input voltage (e.g., V₂), theswitching frequency keeps at the maximum frequency value 396 (e.g., inthe output load range of L₆ and L₇), and the power conversion system 300operates in the DCM mode. Then, the switching frequency decreases fromthe maximum frequency value 396 as the output load increases (e.g., inthe output load range of L₇ and L₈), and the power conversion system 300operates in the QR mode or the valley switching mode, according tocertain embodiments. For example, when the input voltage 370 is at thelow line input voltage (e.g., V₀), the switching frequency decreases inmagnitude (e.g., in the output load range of L₅ and L₈) until reaching aminimum frequency value 398 (e.g., at L₈), and the power conversionsystem 300 operates in the QR mode.

According to yet another embodiment, as shown in FIG. 3(C), if theoutput load is within region V, the primary current 364 continues toincrease in magnitude as the output load increases, and increases fasterwhen the input voltage 370 is close to the low line input voltage thanwhen the input voltage 370 is close to the high line input voltage(e.g., as shown by the waveforms 386 and 388). For example, when theinput voltage 370 is close to the high line input voltage (e.g., betweenV₁ and V₂), the switching frequency continues to decrease as the outputload increases (e.g., as shown by the waveform 382). In another example,as shown in FIG. 3(C) and FIG. 3(D), the system 300 operates in the QRmode or the valley switching mode. On the other hand, when the inputvoltage 370 is close to the low line input voltage (e.g., between V₀ andV₁), the switching frequency keeps at the minimum frequency value 398(e.g., F_(SW_fix)) in some embodiments. For example, as shown in FIG.3(C) and FIG. 3(D), the system 300 operates the CCM mode. The borderbetween region IV and region V varies according to the magnitude of theinput voltage 370 in certain embodiments.

As discussed above and further emphasized here, FIG. 3(B), FIG. 3(C) andFIG. 3(D) are merely examples, which should not unduly limit the scopeof the claims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. For example, as shown inFIG. 3(D), the border between regions I/II/III and region IV (e.g., L₆)and the border between region IV and region V (e.g., L₈) are shown forthe high line input voltage. Such borders may vary with the magnitude ofthe input voltage 370 in certain embodiments. In another example, whenthe output load is within the range of L₆ and L₈, whether the powerconversion system 300 operates in the QR mode depends on the switchingfrequency. In yet another example, when the output load is within therange of L₈ and L₉, whether the power conversion system 300 operates inthe QR mode or the CCM mode depends on the switching frequency.

FIG. 4(A) is a simplified diagram showing certain components of thecontroller 360 as part of the power conversion system 300 according toone embodiment of the present invention. This diagram is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. The controller 360 includes an error amplifier 402, abuffer 404, a sample-and-hold component 406, a demagnetization detector408, a line voltage detector 410, a multi-mode controller 412, a signalconditioning component 414, a filter-and-compensation component 416, apeak-current controller 418, two comparators 420 and 426, a signalprocessing component 422, a flip-flop component 424, a driver 428, asignal generator 430, a slope-compensation component 432, aleading-edge-blanking (LEB) component 434, a cable-drop-compensationcomponent 436, a capacitor 438, and resistors 440, 442, 444 and 446. Inaddition, the controller 360 includes terminals 460, 462, 464, 466 and468.

According to one embodiment, information related to the output voltage326 on the secondary side is extracted through the auxiliary winding 314and the feedback signal 354 is generated based on information related tothe output voltage 326. For example, the controller 360 receives thefeedback signal 354 at terminal 460 (e.g., terminal FB). In anotherexample, the feedback signal 354 is sampled and held by thesample-and-hold component 406. In yet another example, the sampled andheld signal 439 is provided to the error amplifier 402 through at leastthe buffer 404 and the resistor 440 and compared with a reference signal448, and in response, the error amplifier 402 generates a signal 450. Inyet another example, the demagnetization component 408 also receives thefeedback signal 354, and output a signal 483 to the multi-modecontroller 412. In yet another example, if the signal 356 (e.g., PWM) isat a logic high level, the line voltage detector 410 is powered on andreceives a current signal 411 from the terminal 460 (e.g., terminal FB).In yet another example, the line voltage detector 410 outputs a signal484 to the multi-mode controller 412.

According to another embodiment, the error amplifier 402 outputs thesignal 450 to the signal conditioning component 414 which outputs acontrol signal 452 (e.g., EA ctrl) to the multi-mode controller 412 inorder to affect (e.g., select) the operation mode of the powerconversion system 300 (e.g., QR mode, CCM mode, or DCM mode). Forexample, the error amplifier 402 outputs the signal 450 to acompensation network including at least the filter-and-compensationcomponent 416 which outputs a signal 454 to the peak current controller418 in order to affect the primary current 364 of the primary winding302. In another example, the peak current controller 418 generates asignal 456 to the comparator 420 which receives a signal 470 related tothe primary current 364. In yet another example, the comparator 420outputs a signal 472 based on a comparison of the signal 470 and thesignal 456 to the signal processing component 422. Thus the peak valueof the primary current 364 is limited in some embodiments.

For example, the comparator 426 receives a current sensing signal 458related to the primary current 364 through at least the LEB component434 and outputs a signal 474 based on a comparison of the signal 458 anda reference signal 476 to the signal processing component 422. Inanother example, the signal processing component 422 combines thesignals 472 and 474 and outputs a signal 478 to the flip-flop component424 which also receives a signal 480 from the multi-mode controller 412.In yet another example, the flip-flop component 424 outputs a signal 482to the signal generator 430 in order to affect the switching frequencyof the system 300. In yet another example, the driver 428 receives thesignal 482 and outputs the signal 356 to the switch 306. In yet anotherexample, the signal 480 indicates the operation mode of the powerconversion system 300 (e.g., QR mode, CCM mode, or DCM mode).

FIG. 4(B) is a simplified diagram showing certain components of themulti-mode controller 412 as part of the controller 360 according to oneembodiment of the present invention. This diagram is merely an example,which should not unduly limit the scope of the claims. One of ordinaryskill in the art would recognize many variations, alternatives, andmodifications. For example, the multi-mode controller 412 includes acurrent comparator 502, two comparators 504 and 506, two logiccomponents 508 and 518, a QR mode (e.g., CRM mode) component 510, a CCMmode component 512, a valley skipping mode component 514, a frequencyreduction component 516, a DCM and PWM mode component 517, and a drivercomponent 520. In another example, the current comparator 502 isincluded in the line voltage detector 410.

According to one embodiment, the current comparator 502 is activated inresponse to the signal 356. For example, if the signal 356 is at a logichigh level, the current comparator 502 receives a current signal 522related to the input voltage 370 from the terminal 460 (e.g., terminalFB), compares the current signal 522 with a reference current signal524, and outputs a signal 526 that indicates the input voltage 370. Forexample, the control signal 452 (e.g., EA ctrl) are received by thecomparator 504 and the comparator 506, and compared with referencesignals 530 and 532 respectively. In another example, the referencesignal 530 is associated with an upper threshold of the output load ofthe system 300, and the reference signal 532 is associated with a lowerthreshold of the output load of the system 300. In yet another example,the logic component 508 outputs a signal 528 that indicates the outputload of the power conversion system 300.

According to another embodiment, the QR mode (e.g., CRM mode) component510, the CCM mode component 512, the valley skipping mode component 514receive the signal 526, the signal 528 and the signal 452, while thefrequency reduction component 516 and the DCM and PWM mode component 517receive the signal 528. For example, at least one of the QR mode (e.g.,CRM mode) component 510, the CCM mode component 512, the valley skippingmode component 514, the frequency reduction component 516 and the DCMand PWM mode component 517 is activated (e.g., selected) based on atleast information associated with the signal 526, the signal 528 and/orthe signal 452. In another example, when the power conversion system 300is under no/very light load conditions (e.g., region I as shown in FIG.3(B) and FIG. 3(C)), the frequency reduction mode component 516 isactivated (e.g., selected) and the system 300 operates in the DCM modeand the PFM mode. In yet another example, if the power conversion system300 is under light load conditions (e.g., region II as shown in FIG.3(B) and FIG. 3(C)), the DCM and PWM mode component 517 is activated(e.g., selected) and the system 300 operates in the DCM mode and the PWMmode. In yet another example, if the power conversion system 300 isunder medium load conditions (e.g., region III as shown in FIG. 3(B) andFIG. 3(C)), the valley skipping mode component 514 is activated (e.g.,selected), and the system 300 operates in the DCM mode or the valleyskipping mode. In yet another example, when the power conversion system300 is under medium-high load conditions and/or with a high line inputvoltage (e.g., region IV or the top portion of region V as shown in FIG.3(B) and FIG. 3(C)), the QR mode component 510 is activated (e.g.,selected), and the system 300 operates in the QR mode (e.g., the CRMmode or the valley switching mode). In yet another example, when thepower conversion system 300 is under high load conditions with a lowline input voltage (e.g., the bottom portion of region V as shown inFIG. 3(B) and FIG. 3(C)), the CCM mode component 512 is activated (e.g.,selected), and the system 300 operates in the CCM mode or the fixedfrequency mode.

FIG. 5 is a simplified diagram showing certain components of thecontroller 360 as part of the power conversion system 300 according toanother embodiment of the present invention. This diagram is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. The controller 360 includes an error amplifier 602, asample-and-hold component 606, a demagnetization detector 608, amulti-mode controller 612, a signal conditioning component 614, acompensation component 616, a peak-current controller 618, twocomparators 620 and 626, a signal processing component 622, a flip-flopcomponent 624, a driver 628, a signal generator 630, aslope-compensation component 632, a leading-edge-blanking (LEB)component 634, a cable-drop-compensation component 636, a capacitor 638.In addition, the controller 360 includes terminals 660, 662, 664, 666,668 and 670. For example, a compensation capacitor 672 is connected tothe terminal 670.

According to one embodiment, the controller 360 receives the feedbacksignal 354 at the terminal 660 (e.g., terminal FB). In another example,the feedback signal 354 is sampled and held by the sample-and-holdcomponent 606. In yet another example, the sampled and held signal 639is provided to the error amplifier 602 and compared with a referencesignal 648, and in response, the error amplifier 602 generates a signal650 with at least the compensation component 616 that receives thesignals 639 and 648. In yet another example, the demagnetizationcomponent 608 also receives the feedback signal 354, and outputs asignal 684 to the multi-mode controller 612.

According to another embodiment, the signal conditioning component 614receives the signal 650 and outputs a control signal 652 to themulti-mode controller 612 in order to affect the operation mode of thepower conversion system 300 (e.g., QR mode, CCM mode, DCM mode). Forexample, the signal 650 is provided to the current peak controller 618in order to affect the primary current 364 of the primary winding 302.In another example, the peak current controller 618 generates a signal656 to the comparator 620 which receives a signal 670 related to theprimary current 364. In yet another example, the comparator 620 outputsa signal 672 based on a comparison of the signal 670 and the signal 656to the signal processing component 622. Thus the peak value of theprimary current 364 is limited in some embodiments.

For example, the comparator 626 receives a current sensing signal 658related to the primary current 364 through at least the LEB component634 and outputs a signal 674 based on a comparison of the signal 658 anda reference signal 676 to the signal processing component 622. Inanother example, the signal processing component 622 combines thesignals 672 and 674 and outputs a signal 678 to the flip-flop component624 which also receives a signal 680 from the multi-mode controller 612.In yet another example, the flip-flop component 624 outputs a signal 682to the signal generator 630 in order to affect the switching frequencyof the system 300. In yet another example, the driver 628 receives thesignal 682 and outputs the signal 356 to the switch 306.

In addition to voltage regulation as discussed above, the controller 360is implemented for current regulation in some embodiments. FIG. 6 is asimplified diagram showing certain components of the controller 360 aspart of the power conversion system 300 according to yet anotherembodiment of the present invention. This diagram is merely an example,which should not unduly limit the scope of the claims. One of ordinaryskill in the art would recognize many variations, alternatives, andmodifications. The controller 360 includes an error amplifier 702, abuffer 704, sample-and-hold components 701, 703 and 706, ademagnetization detector 708, a multi-mode controller 712, signalconditioning components 714 and 798, a filter-and-compensation component716, a peak-current controller 718, comparators 720, 722 and 726, alogic component 724, a driver 728, a signal generator 730, aslope-compensation component 732, a leading-edge-blanking (LEB)component 734, a cable-drop-compensation component 736, a capacitor 738,resistors 740, 742, 744 and 746, an amplifier 790, switches 792, 794 and796. The sample-and-hold components 701 includes a sampling component795 and a capacitor 705, and the sample-and-hold components 703 includesa sampling component 797 and a capacitor 707. In addition, thecontroller 360 includes terminals 760, 762, 764, 766, 768 and 770. Forexample, a compensation capacitor 771 is connected to the terminal 770.

According to one embodiment, the controller 360 receives the feedbacksignal 354 at terminal 760 (e.g., terminal FB). In another example, thefeedback signal 354 is sampled and held by the sample-and-hold component706. In yet another example, the sampled and held signal 739 is providedto the error amplifier 702 through at least the buffer 704 and theresistor 740 and compared with a reference signal 748, and in response,the error amplifier 702 generates a signal 750. In yet another example,the demagnetization component 708 receives the feedback signal 354, andoutput a demagnetization signal 778 to the multi-mode controller 712.

According to another embodiment, the error amplifier 702 outputs thesignal 750 to the signal conditioning component 714 which outputs acontrol signal 752 to the multi-mode controller 712 in order to affectthe operation mode of the power conversion system 300 (e.g., QR mode,CCM mode, DCM mode). For example, the error amplifier 702 outputs thesignal 750 to a compensation network including at least thefilter-and-compensation component 716 which outputs a signal 754 to thepeak current controller 718 in order to affect the primary current 364of the primary winding 302. In another example, the peak currentcontroller 718 generates a signal 756 to the comparator 720 whichreceives a signal 770 related to the primary current 364. In yet anotherexample, the comparator 720 outputs a signal 772 based on a comparisonof the signal 770 and the signal 756 to the logic component 724. Thusthe peak value of the primary current 364 is limited in someembodiments.

For example, the comparator 726 receives a current sensing signal 758related to the primary current 364 through at least the LEB component734 and outputs a signal 774 based on a comparison of the signal 758 anda reference signal 776 to the logic component 724. In another example,the logic component 724 also receives a signal 775 from the comparator722 and a signal 780 from the multi-mode controller 712, and outputs asignal 782 to the signal generator 730 in order to affect the switchingfrequency of the system 300. In yet another example, the driver 728receives the signal 782 and outputs the signal 356 to the switch 306.

In one embodiment, the sample-and-hold components 701 and 703 sample andhold the current sensing signal 758 at different times. For example, theswitches 792 and 794 are closed or open in response to thedemagnetization signal 778 that indicates the demagnetization process,and the switch 796 is closed or open in response to a complementarysignal of the demagnetization signal 778. In another example, if thesignal 778 indicates that the system 300 operates in the demagnetizationprocess, the switches 792 and 794 are closed and the switch 796 is open.In yet another example, a voltage signal 719 resulting from theredistribution of the charges on the capacitors 705 and 707 is providedto the amplifier 790 during the demagnetization process. In yet anotherexample, the voltage signal 719 is determined as follows:

$\begin{matrix}{V_{in} = \frac{{V_{s\; 1} \times C_{1}} + {V_{s\; 2} \times C_{2}}}{C_{1} + C_{2}}} & \left( {{Equation}\mspace{14mu} 4} \right)\end{matrix}$

where V_(in) represents the voltage signal 719, V_(s1) represents asampled and held signal 709 from the sample-and-hold component 701, andV_(s2) represents a sampled and held signal 711 from the sample-and-holdcomponent 703. In addition, C₁ represents the capacitance of thecapacitor 705, and C₂ represents the capacitance of the capacitor 707.If the capacitance of the capacitor 705 is equal to the capacitance ofthe capacitor 707, the voltage signal 719 is determined as follows, asan example.

$\begin{matrix}{V_{in} = \frac{V_{s\; 1} + V_{s\; 2}}{2}} & \left( {{Equation}\mspace{14mu} 5} \right)\end{matrix}$

In another embodiment, if the signal 778 indicates that the system 300does not operate in the demagnetization process, the switches 792 and794 are open and the switch 796 is closed. For example, a ground voltage721 (e.g., zero) is provided to the amplifier 790. In another example,the amplifier 790 outputs a signal 715 to the signal conditioningcomponent 798 which generates a signal 717 to the comparator 722 inorder to affect the status of the switch 306 and the primary current364.

FIG. 7 is a simplified timing diagram for the power conversion system300 including the controller 360 as shown in FIG. 6 that operates in thecontinuous conduction mode (CCM) according to an embodiment of thepresent invention. This diagram is merely an example, which should notunduly limit the scope of the claims. One of ordinary skill in the artwould recognize many variations, alternatives, and modifications. Thewaveform 802 represents the voltage 358 of the auxiliary winding 314 asa function of time, the waveform 804 represents a secondary current 362flowing through the secondary winding 304 as a function of time, and thewaveform 806 represents the current sensing signal 758 as a function oftime. In addition, the waveform 808 represents an internal samplingsignal of the sample-and-hold component 701 as a function of time, thewaveform 810 represents the signal 356 as a function of time, and thewaveform 812 represents the signal 709 as a function of time. Further,the waveform 814 represents the signal 711 as a function of time, andthe waveform 816 represents a voltage 799 generated by the compensationcapacitor 771.

Three time periods are shown in FIG. 7, including an on-time periodT_(on), a demagnetization time period T_(demag), and an off-time periodT_(off). In the CCM mode, the demagnetization time period T_(demag) isapproximately equal to the off-time period T_(off) in duration. Forexample, T_(on) starts at time t₁₅ and ends at time t₁₆, T_(Demag)starts at the time t₁₆ and ends at time t₁₈, and T_(off) starts at thetime t₁₆ and ends at the time t₁₈. In another example, t₁₅≤t₁₆≤t₁₇≤t₁₈.

According to one embodiment, at the beginning of the on-time period(e.g., at t₁₅), the signal 356 changes from a logic low level to a logichigh level (e.g., a rising edge shown by the waveform 810). For example,in response, a sampling pulse 818 is generated in the internal samplingsignal of the sample-and-hold component 701 (e.g., as shown by thewaveform 808). In another example, during the pulse period of thesampling pulse 818, the sample-and-hold component 701 samples thecurrent sensing signal 758 which increases in magnitude and holds themagnitude 820 of the current sensing signal 758 close to the beginningof the on-time period. In yet another example, the sample-and-holdcomponent 703 samples the current sensing signal 758 during the on-timeperiod T_(on) and holds the magnitude 828 of the current sensing signal758 at the end of the on-time period (e.g., at t₁₆).

According to another embodiment, during the demagnetization period, inresponse to the demagnetization signal 778, the switches 792 and 794 areclosed and the switch 796 is open. For example, the amplifier 790receives a reference signal 713 and the voltage signal 719 (e.g., asshown by the waveforms 812 and 814 respectively), and outputs the signal715. In another example, the following equation is satisfied:

∫(V _(cs_p)(i)+V _(cs_0)(i))×(U(t−T _(s)(i))−U(t−T _(s)(i)−T_(demag)(i))dt−∫V _(ref) dt<a  (Equation 6)

where i represents the i^(th) switching cycle, V_(cs_p)(i) representsthe peak magnitude of the current sensing signal 758 when the switch 306is turned off, and V_(cs_0)(i) represents the magnitude of the currentsensing signal 758 when the switch 306 is turned on. In addition,T_(s)(i) represents the duration of the switching period, T_(demag)(i)represents the duration of the demagnetization period, V_(ref)represents the reference signal 713, U(t) is the unit step function, anda represents a threshold value.

In another example, the following equation can be obtained based onEquation 6:

$\begin{matrix}{{Limit}_{N->\infty}\left( {{\sum\limits_{i = 0}^{N}\; \left( {{V_{{cs}\_ p}(i)} + {{V_{{{cs}\_}0}(i)} \times {T_{demag}(i)}} - {\sum\limits_{i = 0}^{N}\; {V_{ref} \times {T_{s}(i)}}}} \right)} < a} \right.} & \left( {{Equation}\mspace{14mu} 7} \right)\end{matrix}$

where V_(cs_p)=I_(S_P)×R_(S), V_(cs_0)=I_(S_0)×R_(S), I_(S_P) representsthe peak magnitude of the primary current 364 of the primary winding 302when the switch 306 is turned off, I_(S_0) represents the peak magnitudeof the primary current 364 of the primary winding 302 when the switch306 is turned on, and R_(S) represents the resistance of the resistor308. Thus, power delivered to the output load is controlled such thatthe output current is kept approximately constant in some embodiments.

FIG. 8 is a simplified diagram showing certain components of thecontroller 360 as part of the power conversion system 300 according toyet another embodiment of the present invention. This diagram is merelyan example, which should not unduly limit the scope of the claims. Oneof ordinary skill in the art would recognize many variations,alternatives, and modifications. The controller 360 includes an erroramplifier 902, a sample-and-hold component 906, a demagnetizationdetector 908, a multi-mode controller 912, a signal conditioningcomponent 914, a compensation component 916, a peak-current controller918, comparators 920, 922 and 926, a logic component 924, a driver 928,a signal generator 930, a slope-compensation component 932, aleading-edge-blanking (LEB) component 934, a cable-drop-compensationcomponent 936, a capacitor 938. Further, the controller 360 includessample-and-hold components 901 and 903, switches 992, 994 and 996, anamplifier 990, and a signal conditioning component 998. Thesample-and-hold component 901 includes a sampling component 995 and acapacitor 905, and the sample-and-hold component 903 includes a samplingcomponent 997 and a capacitor 907. In addition, the controller 360includes terminals 960, 962, 964, 966, 968 and 970. For example, acompensation capacitor 972 is connected to the terminal 970.

According to one embodiment, the controller 360 receives the feedbacksignal 354 at the terminal 960 (e.g., terminal FB). In another example,the feedback signal 354 is sampled and held by the sample-and-holdcomponent 906. In yet another example, the sampled and held signal 939is provided to the error amplifier 902 and compared with a referencesignal 948, and in response, the error amplifier 902 generates a signal950 with at least the compensation component 916 that receives thesignals 939 and 948. In yet another example, the demagnetizationcomponent 908 also receives the feedback signal 354, and outputs asignal 984 to the multi-mode controller 912.

According to another embodiment, the signal conditioning component 914receives the signal 950 and outputs a control signal 952 to themulti-mode controller 912 in order to affect the operation mode of thepower conversion system 300 (e.g., QR mode, CCM mode, DCM mode). Forexample, the signal 950 is provided to the current peak controller 918in order to affect the primary current 364 of the primary winding 302.In another example, the peak current controller 918 generates a signal956 to the comparator 920 which receives a signal 970 related to theprimary current 364. In yet another example, the comparator 920 outputsa signal 972 based on a comparison of the signal 970 and the signal 956to the logic component 924. Thus the peak value of the primary current364 is limited in some embodiments.

For example, the comparator 926 receives a current sensing signal 958related to the primary current 364 through at least the LEB component934 and outputs a signal 974 based on a comparison of the signal 958 anda reference signal 976 to the logic component 924. In another example,the logic component 924 also receives a signal 975 from the comparator922 and a signal 980 from the multi-mode controller 912 and outputs asignal 982 to the signal generator 930 in order to affect the switchingfrequency of the system 300. In yet another example, the driver 928receives the signal 982 and outputs the signal 356 to the switch 306.

In one embodiment, the sample-and-hold components 901 and 903 sample andhold the current sensing signal 958 at different times. For example, theswitches 992 and 994 are closed or open in response to thedemagnetization signal 984 that indicates the demagnetization process,and the switch 996 is closed or open in response to a complementarysignal of the demagnetization signal 984. In another example, if thesignal 984 indicates that the system 300 operates in the demagnetizationprocess, the switches 992 and 994 are closed and the switch 996 is open.In yet another example, a voltage signal 919 resulting from theredistribution of the charges on the capacitors 905 and 907 is providedto the amplifier 990 during the demagnetization process. In yet anotherexample, the voltage signal 919 is determined as follows:

$\begin{matrix}{V_{in} = \frac{{V_{s\; 1} \times C_{1}} + {V_{s\; 2} \times C_{2}}}{C_{1} + C_{2}}} & \left( {{Equation}\mspace{14mu} 8} \right)\end{matrix}$

where V_(in) represents the voltage signal 919, V_(s1) represents asampled and held signal 909 from the sample-and-hold component 901, andV_(s2) represents a sampled and held signal 911 from the sample-and-holdcomponent 903. In addition, C₁ represents the capacitance of thecapacitor 905, and C₂ represents the capacitance of the capacitor 907.If the capacitance of the capacitor 905 is equal to the capacitance ofthe capacitor 907, the voltage signal 919 is determined as follows, asan example.

$\begin{matrix}{V_{in} = \frac{V_{s\; 1} + V_{s\; 2}}{2}} & \left( {{Equation}\mspace{14mu} 9} \right)\end{matrix}$

In another embodiment, if the signal 984 indicates that thedemagnetization process has completed, the switches 992 and 994 are openand the switch 996 is closed. In yet another example, a ground voltage921 (e.g., zero) is provided to the amplifier 990. In yet anotherexample, the amplifier 990 outputs a signal 915 to the signalconditioning component 998 which generates a signal 917 to thecomparator 922 in order to affect the status of the switch 306 and theprimary current 364.

As discussed above and further emphasized here, FIG. 6, FIG. 7 and FIG.8 are merely examples, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. For example, the waveformsshown in FIG. 7 also apply to the power conversion system 300 with thecontroller 306 as shown in FIG. 8. In another example, the schemes shownin FIG. 7 and FIG. 6 and/or FIG. 8 also apply to the power conversionsystem 300 operating in various modes, including the DCM mode and the QRmode (e.g., CRM mode).

FIG. 9 is a simplified diagram showing certain components of thecontroller 360 as part of the power conversion system 300 according toyet another embodiment of the present invention. This diagram is merelyan example, which should not unduly limit the scope of the claims. Oneof ordinary skill in the art would recognize many variations,alternatives, and modifications. The controller 360 includes an erroramplifier 1002, a sample-and-hold component 1006, a demagnetizationdetector 1008, a multi-mode controller 1012, a signal conditioningcomponent 1014, a compensation component 1016, a peak-current controller1018, comparators 1020, 1022 and 1026, a logic component 1024, a driver1028, a signal generator 1030, a slope-compensation component 1032, aleading-edge-blanking (LEB) component 1034, a cable-drop-compensationcomponent 1036, a capacitor 1038. Further, the controller 360 includes asample-and-hold component 1001, a signal generator 1003, switches 1092and 1096, a gain stage 1094, an amplifier 1090, and a signalconditioning component 1098. The sample-and-hold component 1001 includesa sampling component 1093 and a capacitor 1005. In addition, thecontroller 360 includes terminals 1060, 1062, 1064, 1066, 1068 and 1070.For example, a compensation capacitor 1072 is connected to the terminal1070.

According to one embodiment, the error amplifier 902, thesample-and-hold component 906, the demagnetization detector 908, themulti-mode controller 912, the signal conditioning component 914, thecompensation component 916, the peak-current controller 918, thecomparators 920, 922 and 926, the logic component 924, the driver 928,the signal generator 930, the slope-compensation component 932, theleading-edge-blanking (LEB) component 934, the cable-drop-compensationcomponent 936, the capacitor 938, the switches 992 and 996, theamplifier 990, and the signal conditioning component 998 are the same asthe error amplifier 1002, the sample-and-hold component 1006, thedemagnetization detector 1008, the multi-mode controller 1012, thesignal conditioning component 1014, the compensation component 1016, thepeak-current controller 1018, the comparators 1020, 1022 and 1026, thelogic component 1024, the driver 1028, the signal generator 1030, theslope-compensation component 1032, the leading-edge-blanking (LEB)component 1034, the cable-drop-compensation component 1036, thecapacitor 1038, the switches 1092 and 1096, the amplifier 1090, and thesignal conditioning component 1098, respectively.

According to one embodiment, the controller 360 receives the feedbacksignal 354 at the terminal 1060 (e.g., terminal FB). In another example,the feedback signal 354 is sampled and held by the sample-and-holdcomponent 1006. In yet another example, the sampled and held signal 1039is provided to the error amplifier 1002 and compared with a referencesignal 1048, and in response, the error amplifier 1002 generates asignal 1050 with at least the compensation component 1016 that receivesthe signals 1039 and 1048. In yet another example, the demagnetizationcomponent 1008 also receives the feedback signal 354, and outputs asignal 1084 to the multi-mode controller 1012.

According to another embodiment, the signal conditioning component 1014receives the signal 1050 and outputs a control signal 1052 to themulti-mode controller 1012 in order to affect the operation mode of thepower conversion system 300 (e.g., QR mode, CCM mode, DCM mode). Forexample, the signal 1050 is provided to the current peak controller 1018in order to affect the primary current 364 of the primary winding 302.In another example, the peak current controller 1018 generates a signal1056 to the comparator 1020 which receives a signal 1070 related to theprimary current 364. In yet another example, the comparator 1020 outputsa signal 1072 based on a comparison of the signal 1070 and the signal1056 to the logic component 1024. Thus the peak value of the primarycurrent 364 is limited in some embodiments.

For example, the comparator 1026 receives a current sensing signal 1058related to the primary current 364 through at least the LEB component1034 and outputs a signal 1074 based on a comparison of the signal 1058and a reference signal 1076 to the logic component 1024. In anotherexample, the logic component 1024 also receives a signal 1075 from thecomparator 1022 and a signal 1080 from the multi-mode controller 1012and outputs a signal 1082 to the signal generator 1030 in order toaffect the switching frequency of the system 300. In yet anotherexample, the driver 1028 receives the signal 1082 and outputs the signal356 to the switch 306.

In one embodiment, the signal generator 1003 receives the signal 356 andoutputs a sampling signal 1097 to the sample-and-hold component 1001.For example, in response, the sample-and-hold component 1001 samples thecurrent sensing signal 1058 and holds a magnitude of the current sensingsignal 1058 at a middle point of an on-time period of the switch 306.For example, the sampled and held signal 1009 is provided to the gainstage 1094. In another example, the switch 1092 is closed or open inresponse to the demagnetization signal 1084 that indicates thedemagnetization process, and the switch 1096 is closed or open inresponse to a complementary signal of the demagnetization signal 1084.In yet another example, if the signal 1084 indicates that the system 300operates during the demagnetization process, the switches 1092 is closedand the switch 1096 is open. In yet another example, the gain stage 1094outputs a signal 1095 to the amplifier 1090 through the switches 1092.In yet another example, on the other hand, if the signal 1084 indicatesthat the demagnetization process has completed, the switch 1092 is openand the switch 1096 is closed. In yet another example, a ground voltage1021 (e.g., zero) is provided to the amplifier 1090. In yet anotherexample, the amplifier 1090 outputs a signal 1015 to the signalconditioning component 1098 which generates a signal 1017 to thecomparator 1022 in order to affect the status of the switch 306 and theprimary current 364.

FIG. 10 is a simplified timing diagram for the power conversion system300 including the controller 360 as shown in FIG. 9 that operates in thecontinuous conduction mode (CCM) according to an embodiment of thepresent invention. This diagram is merely an example, which should notunduly limit the scope of the claims. One of ordinary skill in the artwould recognize many variations, alternatives, and modifications. Thewaveform 1102 represents the voltage 358 of the auxiliary winding 314 asa function of time, the waveform 1104 represents a secondary current 362flowing through the secondary winding 304 as a function of time, and thewaveform 1106 represents the current sensing signal 1058 as a functionof time. In addition, the waveform 1108 represents an internal samplingsignal of the sample-and-hold component 1001 as a function of time, thewaveform 1110 represents the signal 356 as a function of time, and thewaveform 1112 represents the signal 1009 as a function of time. Further,the waveform 1114 represents the demagnetization signal 1084 as afunction of time, and the waveform 1116 represents a voltage 1099generated by the compensation capacitor 1072.

Three time periods are shown in FIG. 10, including an on-time periodT_(on), a demagnetization time period T_(demag), and an off-time periodT_(off). In the CCM mode, the demagnetization time period T_(demag) isapproximately equal to the off-time period T_(off) in duration. Forexample, T_(on) starts at time t₁₉ and ends at time t₂₁, T_(Demag)starts at the time t₂₁ and ends at time t₂₂, and T_(off) starts at thetime t₂₁ and ends at the time t₂₂. In another example, t₁₉≤t₂₀≤t₂₁≤t₂₂.

According to one embodiment, at the beginning of the on-time period(e.g., at t₁₉), the sampling signal 1097 changes from a logic low levelto a logic high level. For example, at the middle point of the on-timeperiod (e.g., at t₂₀), the sampling signal 1097 changes from the logichigh level to the logic low level (e.g., a falling edge as shown by thewaveform 1108). In another example, in response, the sample-and-holdcomponent 1001 samples the current sensing signal 1158 and holds amagnitude 1118 of the current sensing signal 1158 (e.g., as shown by thewaveforms 1108 and 1112). In yet another example, the magnitude 1118 isdetermined as follows:

$\begin{matrix}{V_{{{cs}\_}\frac{1}{2}T_{on}} = {{I_{{S\_}\frac{1}{2}{Ton}} \times R_{S}} = \frac{V_{{cs}\_ p} + V_{{{cs}\_}0}}{2}}} & \left( {{Equation}\mspace{14mu} 10} \right)\end{matrix}$

where V_(cs_p) represents the peak magnitude of the current sensingsignal 1058 when the switch 306 is turned off during the switchingcycle, and V_(cs_0) represents the magnitude of the current sensingsignal 1058 when the switch 306 is turned on during the switching cycle.

According to another embodiment, during the demagnetization period, inresponse to the demagnetization signal 1084, the switch 1092 is closedand the switch 1096 is open. For example, the gain stage 1094 outputsthe signal 1095 to the amplifier 1090 through the switch 1092, and theamplifier 1090 outputs the signal 1015. In another example, thefollowing equation is satisfied:

$\begin{matrix}{G \times {\int{{V_{{{cs}\_}\frac{1}{2}T_{on}}(i)} \times \left( {{{U\left( {t - {T_{s}(i)}} \right)} - {{U\left( {t - {T_{s}(i)} - {T_{demag}(i)}} \right)}{dt}} - {\int{V_{reF}{dt}}}} < a} \right.}}} & \left( {{Equation}\mspace{14mu} 11} \right)\end{matrix}$

where i represents the i^(th) switching cycle,

$V_{{{cs}\_}\frac{1}{2}T_{on}}(i)$

represents the magnitude of the current sensing signal 1058 at a middlepoint of an on-time period during the i^(th) switching cycle, andT_(s)(i) represents the duration of the switching period. In addition,T_(demag)(i) represents the duration of the demagnetization period,V_(ref) represents the reference signal 1013, and G represents a ratio.

In another example, the following equation can be obtained based onEquation 11:

$\begin{matrix}{{{Limit}_{N->\infty}\left( {{\sum\limits_{i = 0}^{N}\; {G \times {V_{{{cs}\_}\frac{1}{2}T_{on}}(i)} \times {T_{demag}(i)}}} - {\sum\limits_{i = 0}^{N}\; {V_{ref} \times {T_{s}(i)}}}} \right)} < a} & \left( {{Equation}\mspace{14mu} 12} \right)\end{matrix}$

Thus, power delivered to the output load is controlled such that theoutput current is kept approximately constant in some embodiments. Forexample, if G=2, Equation 7 becomes the same as Equation 4, and Equation8 becomes the same as Equation 5.

According to another embodiment, a system controller for regulating apower conversion system includes an operation-mode-selection componentand a driving component. The operation-mode-selection component isconfigured to receive a first signal related to an output load of thepower conversion system and a second signal related to an input signalreceived by the power conversion system and output a mode-selectionsignal based on at least information associated with the first signaland the second signal. The driving component is configured to receivethe mode-selection signal and generate a drive signal based on at leastinformation associated with the mode-selection signal, the drivingsignal corresponding to a switching frequency. Theoperation-mode-selection component is further configured to, if theoutput load is determined to be larger than a first load threshold inmagnitude and the input signal is determined to be larger than an inputthreshold, generate the mode-selection signal corresponding to aquasi-resonant mode if the switching frequency is determined to besmaller than a first frequency threshold and larger than a secondfrequency threshold. In addition, the operation-mode-selection componentis configured to, if the output load is determined to be larger than thefirst load threshold in magnitude and the input signal is determined tobe smaller than the input threshold, generate the mode-selection signalcorresponding to a continuous conduction mode if the switching frequencyis determined to be equal to the second frequency threshold. Forexample, the system controller is implemented according to at least FIG.3(A), FIG. 3(B), FIG. 3(C), FIG. 4(A), FIG. 4(B) and/or FIG. 5.

According to yet another embodiment, a system controller for regulatinga power conversion system includes a driver component, a firstsample-and-hold component, a second sample-and-hold component, a firstswitch, a second switch, and a signal processing component. The drivercomponent is configured to output a drive signal in order to affect anoutput signal of the power conversion system, the drive signal beingassociated with a switching frequency corresponding to a switchingperiod, the switching period including an on-time period and ademagnetization period. The first sample-and-hold component includes afirst capacitor and is configured to sample and hold a current sensingsignal associated with a primary current flowing through a primarywinding of the power conversions system at at least a first time duringthe on-time period and generate a first held sampled signal based on atleast information associated with the current sensing signal. The secondsample-and-hold component includes a second capacitor and is configuredto sample and hold the current sensing signal at at least a second timeduring the on-time period and generate a second held sampled signalbased on at least information associated with the current sensingsignal, the second time being later than the first time. The firstswitch includes a first switch terminal and a second switch terminal,the first switch terminal being coupled to the first capacitor, thesecond switch terminal being coupled to the second capacitor, the firstswitch being further configured to be closed during the demagnetizationperiod and open during the on-time period. The second switch includes athird switch terminal and a fourth switch terminal, the third switchterminal being coupled to the first switch terminal, the second switchbeing further configured to be closed during the demagnetization periodand open during the on-time period. The signal processing component isconfigured to receive a combined signal from the fourth switch terminalif the first switch and the second switch are closed and output aprocessed signal based on at least information associated with thecombined signal to the driver component. For example, the systemcontroller is implemented according to at least FIG. 6, FIG. 7 and/orFIG. 8.

According to yet another embodiment, a system controller for regulatinga power conversion system includes a driver component, a sample-and-holdcomponent, a switch, a signal processing component, and anoperation-mode-selection component. The driver component is configuredto output a drive signal in order to affect an output signal of thepower conversion system, the drive signal being associated with aswitching frequency corresponding to a switching period, the switchingperiod including an on-time period and a demagnetization period. Thesample-and-hold component includes a first capacitor and is configuredto sample and hold a current sensing signal associated with a primarycurrent flowing through a primary winding of the power conversionssystem at a middle point of the on-time period and generate a heldsampled signal based on at least information associated with the currentsensing signal. The switch includes a first switch terminal and a secondswitch terminal, the first switch terminal being coupled to the firstcapacitor, the switch being further configured to be closed during thedemagnetization period and open during the on-time period. The signalprocessing component is configured to receive a third signal from thesecond switch terminal if the switch is closed and output a processedsignal based on at least information associated with the third signal tothe driver component. The operation-mode-selection component isconfigured to receive a first signal related to an output load of thepower conversion system and a second signal related to an input signalreceived by the power conversion system and output a mode-selectionsignal based on at least information associated with the first signaland the second signal. The driver component is further configured toreceive the mode-selection signal and generate the drive signal based onat least information associated with the mode-selection signal. Forexample, the system controller is implemented according to at least FIG.9, and/or FIG. 10.

In one embodiment, a method for regulating a power conversion systemincludes receiving a first signal related to an output load of the powerconversion system and a second signal related to an input signalreceived by the power conversion system, processing informationassociated with the first signal and the second signal, and generating amode-selection signal based on at least information associated with thefirst signal and the second signal. In addition, the method includesreceiving the mode-selection signal, processing information associatedwith the mode-selection signal, and generating a drive signal based onat least information associated with the mode-selection signal. Theprocess for generating a mode-selection signal based on at leastinformation associated with the first signal and the second signalincludes if the output load is determined to be larger than a first loadthreshold in magnitude and the input signal is determined to be largerthan an input threshold, generating the mode-selection signalcorresponding to the quasi-resonant mode if the switching frequency isdetermined to be smaller than a first frequency threshold and largerthan a second frequency threshold. The process for generating amode-selection signal based on at least information associated with thefirst signal and the second signal further includes if the output loadis determined to be larger than the first load threshold in magnitudeand the input signal is determined to be smaller than the inputthreshold, generating the mode-selection signal corresponding to thecontinuous conduction mode if the switching frequency is determined tobe equal to the second frequency threshold. For example, the method isimplemented according to at least FIG. 3(A), FIG. 3(B), FIG. 3(C), FIG.4(A), FIG. 4(B) and/or FIG. 5.

In another embodiment, a method for regulating a power conversion systemincludes generating a drive signal in order to affect an output signalof the power conversion system, the drive signal being associated with aswitching frequency corresponding to a switching period, the switchingperiod including an on-time period and a demagnetization period, andsampling and holding, by at least a first sample-and-hold component, atat least a first time during the on-time period, a current sensingsignal in order to generate a first held sampled signal, the currentsensing signal being associated with a primary current flowing through aprimary winding of the power conversions system, the firstsample-and-hold component including a first capacitor. The methodfurther includes sampling and holding, by at least a secondsample-and-hold component, at at least a second time during the on-timeperiod, the current sensing signal in order to generate a second heldsampled signal, the second sample-and-hold component including a secondcapacitor, the second time being later than the first time. In addition,the method includes generating a combined signal during thedemagnetization period by at least a first switch, the first switchincluding a first switch terminal coupled to the first capacitor and asecond switch terminal coupled to the second capacitor, receiving thecombined signal by at least a second switch including a third switchterminal and a fourth switch terminal, the third switch terminal beingcoupled to the first switch terminal, and outputting a processed signalbased on at least information associated with the combined signal duringthe demagnetization period. For example, the method is implementedaccording to at least FIG. 6, FIG. 7 and/or FIG. 8.

In yet another embodiment, a method for regulating a power conversionsystem includes receiving a first signal related to an output load ofthe power conversion system and a second signal related to an inputsignal received by the power conversion system, processing informationassociated with the first signal and the second signal, and generating amode-selection signal based on at least information associated with thefirst signal and the second signal. The method further includesreceiving the mode-selection signal, processing information associatedwith the mode-selection signal, and generating the drive signal based onat least information associated with the mode-selection signal in orderto affect an output signal of the power conversion system, the drivesignal being associated with a switching frequency corresponding to aswitching period, the switching period including an on-time period and ademagnetization period. In addition, the method includes sampling andholding, by at least a sample-and-hold component, at a middle point ofthe on-time period, a current sensing signal in order to generate a heldsampled signal, the current sensing signal being associated with aprimary current flowing through a primary winding of the powerconversions system, the sample-and-hold component including a capacitor,receiving a third signal during the demagnetization period through atleast a switch coupled to the capacitor, and outputting a processedsignal based on at least information associated with the third signalduring the demagnetization period. For example, the method isimplemented according to at least FIG. 9, and/or FIG. 10.

For example, some or all components of various embodiments of thepresent invention each are, individually and/or in combination with atleast another component, implemented using one or more softwarecomponents, one or more hardware components, and/or one or morecombinations of software and hardware components. In another example,some or all components of various embodiments of the present inventioneach are, individually and/or in combination with at least anothercomponent, implemented in one or more circuits, such as one or moreanalog circuits and/or one or more digital circuits. In yet anotherexample, various embodiments and/or examples of the present inventioncan be combined.

Although specific embodiments of the present invention have beendescribed, it will be understood by those of skill in the art that thereare other embodiments that are equivalent to the described embodiments.Accordingly, it is to be understood that the invention is not to belimited by the specific illustrated embodiments, but only by the scopeof the appended claims.

1. A system controller for regulating a power conversion system, thesystem controller comprising: an operation-mode-selection componentconfigured to receive a first signal related to an output load of thepower conversion system and a second signal related to an input signalreceived by the power conversion system and output a mode-selectionsignal based on at least information associated with the first signaland the second signal; and a driving component configured to receive themode-selection signal and generate a drive signal based on at leastinformation associated with the mode-selection signal, the drivingsignal corresponding to a switching frequency; wherein theoperation-mode-selection component is further configured to: if theoutput load is determined to be larger than a first load threshold inmagnitude and the input signal is determined to be larger than an inputthreshold, generate the mode-selection signal corresponding to aquasi-resonant mode if the switching frequency is determined to besmaller than a first frequency threshold and larger than a secondfrequency threshold; and if the output load is determined to be largerthan the first load threshold in magnitude and the input signal isdetermined to be smaller than the input threshold, generate themode-selection signal corresponding to a continuous conduction mode ifthe switching frequency is determined to be equal to the secondfrequency threshold.
 2. The system controller of claim 1 wherein theoperation-mode-selection component is further configured to receive thefirst signal from a terminal during a first time period and receive thesecond signal from the terminal during a second time period.
 3. Thesystem controller of claim 2 wherein the first time period and thesecond time period are different.
 4. The system controller of claim 2wherein: the first signal is a voltage signal; and the second signal isa current signal.
 5. The system controller of claim 2, and furthercomprising: a signal processing component configured to receive afeedback signal and generate the first signal based on at leastinformation associated with the feedback signal; and a signal detectorconfigured to generate the second signal based on at least informationassociated with the input signal.
 6. The system controller of claim 5wherein the signal processing component includes: a sampling componentconfigured to receive the feedback signal and output a sampled signalbased on at least information associated with the feedback signal; anerror amplifier configured to receive the sampled signal and output anerror signal related to the first signal based on at least informationassociated with the sampled signal.
 7. The system controller of claim 1wherein the operation-mode-selection component is further configured to,if the output load is determined to be smaller than the first loadthreshold and larger than a second load threshold in magnitude, generatethe mode-selection signal corresponding to the quasi-resonant mode ifthe switching frequency is determined to be smaller than the firstfrequency threshold and larger than the second frequency threshold. 8.The system controller of claim 7 wherein the operation-mode-selectioncomponent is further configured to, if the output load is determined tobe smaller than the first load threshold and larger than the second loadthreshold in magnitude, generate the mode-selection signal correspondingto a discontinuous conduction mode if the switching frequency isdetermined to be equal to the first frequency threshold.
 9. The systemcontroller of claim 7 wherein the operation-mode-selection component isfurther configured to, if the output load is determined to be smallerthan the second load threshold in magnitude, generate the mode-selectionsignal corresponding to a discontinuous conduction mode.
 10. The systemcontroller of claim 9 wherein the discontinuous conduction mode is oneselected from a group consisting of a valley skipping mode, a frequencymodulation mode, and a pulse-width-modulation mode.
 11. The systemcontroller of claim 10 wherein the operation-mode-selection controllerincludes a discontinuous-conduction-mode selector configured to receivethe first signal and, if the output load is determined to be smallerthan the second load threshold in magnitude, generate the mode selectionsignal corresponding to the discontinuous conduction mode.
 12. Thesystem controller of claim 7 wherein the second load threshold varieswith the input signal in magnitude
 13. The system controller of claim 1wherein the operation-mode-selection controller includes a comparatorconfigured to receive the second signal and a reference signal andoutput a comparison signal based on at least information associated withthe second signal and the reference signal.
 14. The system controller ofclaim 13 wherein the operation-mode-selection controller includes acontinuous-conduction-mode selector configured to receive the firstsignal and the comparison signal and, if the output load is determinedto be larger than the first load threshold in magnitude and the inputsignal is determined to be smaller than the input threshold, generatethe mode selection signal corresponding to the continuous conductionmode.
 15. The system controller of claim 13 wherein theoperation-mode-selection controller includes a quasi-resonant-modeselector configured to: receive the first signal and the comparisonsignal; if the output load is determined to be larger than the firstload threshold in magnitude and the input signal is determined to belarger than the input threshold, generate the mode-selection signalcorresponding to the quasi-resonant mode; and if the output load isdetermined to be smaller than the first load threshold and larger than asecond load threshold in magnitude, generate the mode-selection signalcorresponding to the quasi-resonant mode.
 16. The system controller ofclaim 1 wherein the input signal is an input line voltage.
 17. Thesystem controller of claim 1 wherein the driving component includes: aflip-flop component configured to receive the mode-selection signal andoutput a frequency modulation signal based on at least informationassociated with the mode-selection signal to theoperation-mode-selection component; and a driver configured to receivethe frequency modulation signal and output the drive signal based on atleast information associated with the frequency modulation signal. 18.The system controller of claim 1 wherein the quasi-resonant mode is oneselected from a group consisting of a valley switching mode, and acritical conduction mode.
 19. The system controller of claim 1, andfurther comprising a primary-side sensing component configured toreceive a feedback signal from an auxiliary winding of the powerconversion system.
 20. The system controller of claim 1 wherein thefirst load threshold varies with the input signal in magnitude
 21. Thesystem controller of claim 1 wherein: the input signal is larger than athird threshold in magnitude and smaller than a fourth threshold inmagnitude; and the input threshold is larger than the third thresholdand smaller than the fourth threshold. 22.-46. (canceled)
 47. A methodfor regulating a power conversion system, the method comprising:receiving a first signal related to an output load of the powerconversion system and a second signal related to an input signalreceived by the power conversion system; processing informationassociated with the first signal and the second signal; generating amode-selection signal based on at least information associated with thefirst signal and the second signal; receiving the mode-selection signal;processing information associated with the mode-selection signal; andgenerating a drive signal based on at least information associated withthe mode-selection signal; wherein the process for generating amode-selection signal based on at least information associated with thefirst signal and the second signal includes: if the output load isdetermined to be larger than a first load threshold in magnitude and theinput signal is determined to be larger than an input threshold,generating the mode-selection signal corresponding to the quasi-resonantmode if the switching frequency is determined to be smaller than a firstfrequency threshold and larger than a second frequency threshold; and ifthe output load is determined to be larger than the first load thresholdin magnitude and the input signal is determined to be smaller than theinput threshold, generating the mode-selection signal corresponding tothe continuous conduction mode if the switching frequency is determinedto be equal to the second frequency threshold.
 48. The method of claim47, and further comprising: receiving a feedback signal by aprimary-side sensing component from an auxiliary winding of the powerconversion system.
 49. The method of claim 47 wherein the process forgenerating a mode-selection signal further includes: if the output loadis determined to be smaller than the first load threshold and largerthan a second load threshold in magnitude, generating the mode-selectionsignal corresponding to the quasi-resonant mode if the switchingfrequency is determined to be smaller than the first frequency thresholdand larger than the second frequency threshold; and generating themode-selection signal corresponding to a discontinuous conduction modeif the switching frequency is determined to be equal to the firstfrequency threshold. 50.-53. (canceled)